(āt-mŏm'ĭ-tər) An instrument that measures the rate of evaporation of water into the atmosphere. Atmometers usually measure the evaporation of water from a free water surface (such as a pan of water set into the ground so that the water's surface is even with the ground's surface) or from a porous, water-saturated surface (such as filter paper placed over a graduated cylinder of water). In the first type, the difference between the ground surface and the water level is used to calculate the volume of water that has evaporated in a given period of time. In the second type, the water volume is read directly from the graduated cylinder. Also called evaporimeter.